/*! \page TimeKeeping "Time Keeping"
\tableofcontents
Design document for date and time handling in KStars
\section Basics The Basics

Timekeeping is handled by the SimClock class.  SimClock stores the
simulation time as the Julian Day, in a long double variable
("julian").  A long double is required to provide sub-second resolution
in the Julian Day value.  The date can be converted to a calendar date
(QDateTime object) with the UTC() function.  julian is updated every
0.1 sec by an internal QTimer, using the SimClock::tick() SLOT,
connected to the internal QTimer's timeout() SIGNAL.

We make a distinction between "system time" and "simulation time".
System time is real time, according to the computer's CPU clock.
Simulation time is the time according to KStars; since the time and
date are adjustable, system time and simulation time can have an
arbitrary offset.  Furthermore, SimClock has an adjustable Scale
parameter that determines how many seconds of simulation time pass for
each second of system time.  Scale can even be negative, indicating
that the simulation clock is running backwards.

The simplest way to advance the simulation time would be to add
(0.1*Scale) seconds to julian every time tick() is called.  However,
this is not accurate, because there is always some error associated
with the time it takes to execute tick(), and these errors would
accumulate during each cycle.  Instead, tick() measures the elapsed
time since some fixed system-time marker ("sysmark"), and adds
(elapsed_time*Scale) seconds to "julianmark", a fixed simulation-time
marker that was the exact simulation time at the moment the system-time
marker was set.  This is much more accurate, because any errors in
tick() do not accumulate.  Any time the clock is started, or its
scale changed, the sysmark and julianmark markers are reset (they are
also reset if they have not changed in more than 24 hours of real time).

tick() emits the timeAdvanced() signal, which is connected to
KStarsData::updateTime(), which takes care of updating object
coordinates and drawing the skymap (see below for details).

Note also that the SimClock class only handles the Julian Day and the
Universal Time, not the local time.  Time zone corrections and daylight
savings time are handled by KStarsData::updateTime().

\section Manual Manual Mode

The above procedure works well, as long as tick() takes less than 0.1 sec,
on average (including the time taken by KStarsData::updateTime()).  In
practice, large values of Scale cause more calls to updateTime() than the
CPU is able to handle.  This results in some time steps being skipped
altogether, which makes the simulation seem jerky.

To compensate for this, we implemented a "Manual Mode" for SimClock.  In
Manual mode, the internal QTimer is stopped, so that tick() is not
triggered every 0.1 seconds.  Instead, a similar function (manualTick())
is called whenever KStarsData::updateTime() has finished.  manualTick()
adds Scale seconds to the simulation time.  So, the Scale parameter has
a slightly different meaning in Manual mode.  The simulation time
no longer runs at strictly Scale seconds per real-time second; rather,
every update of the simulation occurs exactly Scale simulation-seconds
after the previous update, no matter how long the update takes.

There are two bool variables in SimClock, ManualMode and ManualActive.
The first controls whether the clock is using Manual Mode (accessed by
isManualMode()); the second controls whether the clock is running in
Manual Mode (recall that the internal timer is halted when in Manual
Mode).  The function isActive() returns whether the clock is running,
for both the standard mode and Manual Mode.


\section UpdateTime KStarsData::updateTime()

updateTime() is a SLOT connected to the SimClock's timeAdvanced()
SIGNAL, which is emitted every tick() or manualTick().

KStarsData keeps its own representation of the universal time as a
QDateTime object (UTime); the first thing that updateTime() does is to
reset this with clock->UTC().  It then sets the local time QDateTime
object (LTime) by adding 3600*geo->TZ() seconds to UTime.  It then
checks if it has reached the next daylight savings time change point,
and adjusts the Time Zone offset, if necessary.

There is a group of time-dependent numbers such as the obliquity and
the sun's mean anomaly; these are kept in the KSNumbers class.  The next
thing updateTime() does is create a KSNumbers object appropriate for the
current julian day value [we may be able to save some time by keeping a
persistent KSNumbers object, and not updating it on every call to
updateTime(), as the values stored there don't change very quickly].

There are several things that don't need to be updated on every call to
updateTime().  To save time, we only update them if a certain amount of
time has passed since the last update.  For example, the LastNumUpdate
variable stores the julian day of the last time object coordinates were
updated for precession/nutation/aberration.  This needs to happen once
per simulation day, so whenever (CurrentDate-LastNumUpdate) exceeds 1.0,
it signals the update (by setting needNewCoords=true) and resets
LastNumUpdate to CurrentDate.  Similarly, we use LastPlanetUpdate to
update planet coordinates 100 times per day.  LastSkyUpdate monitors
the last time the horizontal coordinates were updated (the update
interval is dependent on the current zoom setting).

Next, we update the focus position.  If no object is being tracked, and
useAltAz=true, then the focus RA needs to advance at the sidereal rate
(one second on the sky per sidereal second of time).  If the simulation
is tracking an object, then the focus is set to the object's coordinates.

Finally, the last thing updateTime() does is to re-draw the sky by calling
SkyMap::update().

*/
